Endurance exercise training has been suggested to alter the autonomic nervous system resulting in an apparent increase in parasympathetic efferent activity and a decrease in sympathetic efferent activity. In this context, it is interesting to speculate that exercise training may result in an altered neurohumoral regulation of the cardiovascular system by changing the interaction of systems directly involved in controlling the circulation. Changes in autonomic neural activity could be beneficial to patients at high risk for sudden cardiac death. For example, a reduction in the incidence of sudden cardiac death among postinfarction patients participating in a multifactorial intervention program that included exercise has been reported. This proposal is designed to examine the effects of endurance exercise training on the cardiopulmonary baroreflex regulation of the cardiovascular system. Specifically, the direct inhibitory influence of the cardiopulmonary baroreflex and the interaction of the cardiopulmonary and arterial baroreflex on the regulation of sympathetic nerve activity (lumbar and renal), regional blood flow, arterial pressure, heart rate and hormonal regulation (renin and arginine vasopressin) at rest and during exercise will be studied both before and after exercise training. The influence of peripheral (afferent activity) and central mechanisms in mediating the autonomic adaptations associated with exercise training will also be examined. The underlying hypothesis of this proposal is that the tonic inhibitory influence of cardiopulmonary baroreflex on the sympathetic nervous system, arterial baroreflex and hormonal regulation is enhanced as a result of endurance exercise training and that this response is mediated by both peripheral and central mechanisms. An enhanced cardiopulmonary reflex may significantly alter the integrative regulation of the cardiovascular system by initiating reflex alterations in the neurohumoral drive to the circulation as well as interacting with the arterial baroreflexes. These studies will provide a better understanding of the regulation of the neurohumoral drive to the circulation, the effect of exercise training on this regulation and the influence of central and peripheral mechanisms in mediating the autonomic adaptations associated with exercise training.